Epoxy resin compositions containing aromatic biguanides as latent curing agents therefor

ABSTRACT

NEW COMPOUNDS USEFUL AS LATENT CURING AGENTS FOR EPOXY RESINS ARE PREPARED BY REACTING AN AROMATIC DIAMINE, E.G. METHYLENEDIANILINE, WITH DICANDIAMIDE. THESE LATENT CURING AGENTS WILL CURE EPOXY RESIBNS WITH OR WITHOUT ACCELERATORS IN SHORTER TIMES THAN ACCELERATED DICYANDIAMIDE CURED SYSTEMS AND AT LOWER TEMPERATURES THAN INACELLERATED DICYANIDIAMIDE SYSTEMS.

United States Patent D 3,795,657 EPOXY RESIN COMPOSITIONS CONTAINING ARO- MATIC BIGUANIDES AS LATENT CURING AGENTS THEREFOR Robert William Howsam and Paul Alan Larson, Lake Jackson, Tex., assignors to The Dow Chemical Company, Midland, Mich. No Drawing. Filed Aug. 21, 1972, Ser. No. 282,494 Int. Cl. C08g 30/14 US. Cl. 260-47 EN 10 Claims ABSTRACT OF THE DISCLOSURE New compounds useful as latent curing agents for epoxy resins are prepared by reacting an aromatic diamine, e.g. methylenedianiline, with dicyandiamide. These latent curing agents will cure epoxy resins with or without accelerators in shorter times than accelerated dicyandiamide cured systems and at lower temperatures than unaccelerated dicyandiamide systems.

This invention relates to new latent curing agents for epoxy resins, and to epoxy resins in admixture with such curing agents and the cured products thereof.

Dicyandiamide is a well known latent curing agent for epoxy resins. To obtain adequate cure in desirable and practical times and temperatures, however, the dicyandiamide-epoxy resin mixtures must be used with an accelerator.

It has now been discovered that the biguanides, formed by reacting aromatic diamines with dicyandiamide, will cure epoxy resins at lower temperatures than dicyandiamide when using equivalent amounts of accelerators. Without added accelerators these biguanides will cure epoxy resins faster than accelerated dicyandiamide at the same temperatures. Mixtures of these biguanides with epoxy resins when formulated as paints or varnishes are quite stable over extended periods of time and maintain their curability.

The new latent curing agents are represented by the general formulae wherein A is a divalent hydrocarbon radical having from 1 to about 6 carbon atoms,

each X is independently hydrogen, chlorine or bromine, and x has a value of or 1.

The new curing agents of the present invention are prepared by forming the hydrohalide salt of the desired aromatic diamine by reaction of said aromatic diamine with a hydrogen halide at a temperature of from about 0 to about 50 C. in an aqueous medium and subsequently reacting the resultant aromatic diamine hydrohalide salt with dicyandiamide in a molar ratio of at least 2 moles of dicyandiamide per 1 mole of aromatic diamine hydrohalide at a temperature of from about 90 to about 100 3,795,657 Patented Mar. 5, 1974 C. for a time suflicient to complete the reaction. Then a stoichiometric excess of an alkali metal hydroxide or alkaline earth metal hydroxide is added to neutralize the hydrohalide salt thereby producing the desired product which may be recovered by washing with water and subsequently with an alcohol such as, for example, ethanol or methanol.

Suitable aromatic diamines which may be reacted with dicyandiamide to prepare the latent curing agents of the present invention may be represented by the general formulae (III) NH: (A): NH:

X X and HzN NH:

wherein A, X, and x are as defined in Formulae I and II and such diamines include, for example,

m-phenylenedlamine@ I 0 bis (aminophenyl) sulfone@ g HzN A N H:

NH: bis (amlnophenyl) oxide@0 HzN NH: bis (aminophenyl) sulfide@ S HzN bis(aminophenyl)sulfoxide@- f1) NHz, bis (amiuophenyl) ketone@C-@ bis (aminophenyl)dlsulfide@ S S HzN N H:

and the like and mixtures thereof.

The curable compositions of the present invention comprise mixtures of (A) an epoxy resin of the polyglycidyl ether type having more than one 1,2-epoxy group and (B) one or more of the compounds represented by Formulae I and II;

wherein the ratio of BzA is such as to provide an amine hydrogen: epoxide equivalent ratio of from about 0.8:1 to about 2:1 and preferably from about 1:1 to about 1.621.

The curable compositions of the present invention may also contain various modifiers such as fillers, extenders, pigments, accelerators and other modifiers such as carboxyl-containing polymers, urethanes, mixtures of any of such modifiers and the like. They may also be modified to contain any of the other well known curing agents such as amines, and anhydrides, imidazoles and the like. For example, accelerators useful with dicyandiamide, such as Z-methylimidazole, benzyl dimethylamine, chlorophenyl 3,3-dimethyl urea and others may be employed with the biguanide curing agents. The curable compositions are useful in the preparation of adhesives, laminates, powder coatings, castings and the like.

EXAMPLE 1 Preparation of methylenedianiline-dicyandiamide adduct (MDA/DICY) To a stainless steel reactor equipped with a stirrer, reflux condenser and temperature control means containing 35 lbs. of water was slowly added 15.5 lbs. of concentrated HCl. Then 15.3 lbs. of methylenedianiline was added and the temperature raised to 100 C. and maintained thereat until the methylenedianiline had dissolved. When the methylenedianiline had dissolved, 13 lbs. of dicyandiamide. was added and after refluxing (100 C.) for two hours, 100 lbs. of 20 C. water was added followed by the addition of 155 lbs. of 5% aqueous NaOH. After stirring for about 5 minutes, the product was centrifuged, washed with 40 lbs. of water, then centrifuged and washed again with 40 lbs. of water. After contrifuging again, the product was washed with 20 lbs. of methanol and centrifuged and washed again with 20 lbs. of methanol. After centrifuging and drying at 80 C. under vacuum, twelve lbs. of product (40% yield) of a white solid having a melting point of 210 C., an amine hydrogen equivalent weight of 33.8 was recovered.

EXAMPLE 2 Preparation of methylene di-o-chloro aniline-dicyandiamide adduct (MOCA/DICY) To a glass reaction vessel containing 300 ml. of water, equipped with a stirrer, reflux condenser and temperature control means, 104 grams of concentrated HCl was slowly added. Then 133.5 grams (0.5 mole) of methylene di-o chloroaniline (MOCA) was added and the temperature raised to about 100 C. and maintained thereat until the MOCA had dissolved. After the MOCA had dissolved, 84 grams (1 mole) of dicyandiamide was added and the contents were refluxed (100 C.) for about twenty minutes. The contents were cooled by the addition of water in an amount of 2.5 times the original volume and the contents poured into a container containing 1000 ml. of 5% aqueous NaOH. The white precipitate was washed twice with 500 ml. of water to remove salt, caustic and any unreacted dicyandiamide. The white powder was then washed with 2000 ml. of ethanol followed by 500 ml. of methylene chloride until the filtrate was no longer discolored. About 90 grams (41% yield) of a white powder was obtained which had a melting point of 215 C. and an amine hydrogen equivalent weight of 35.2. The equivalent weight was calculated from the elemental analysis and based on nitrogen content.

4 EXAMPLE 3 Preparation of metaphenylene diamine-dicyandiamide adduct (MPDA/DICY) To a reaction vessel equipped with a stirrer, reflux condenser and temperature control means and containing grams of water and 100 grams concentrated HCl was dissolved 53 grams (0.5 mole) of m-phenylenediamine. The solution was heated and 84 grams (1.0 mole) of dicyandiamide was added and the contents refluxed for 2.5 hours. The solution was cooled with 50 ml. of water and poured into 5 0 ml. of a 25% aqueous solution of NaOH. stirred and filtered and grams of crude product was recovered.

The crude product, which weighed 40 grams, was refluxed in 50 ml. of a 4:1 by volume mixture of ethanol and water for 1 to 2 minutes, then cooled, filtered and washed with ethanol and then methylene chloride, as in Example 2 above, until a clear filtrate was obtained. After being dried in an oven at 50 C., the pure white solid product, which weighed 20.6 grams, had a melting point of 172 C. and an amine hydrogen equivalent weight of 22.7 calculated from the elemental analysis.

EXAMPLE 4 An epoxy resin adhesive was formulated from an MDA/DICY adduct prepared in a manner similar to Example 1. For comparative purposes, an adhesive was also prepared using the same epoxy resin with dicyandiamide. The composition of the adhesives are given in Table I and the properties are given in Table II.

1 Urethane modified DGEBA is a diglycidyl ether of bisphenol A having an EEW of about 189 modified with 4.2 phr. tetraethylene glycerol 20 phr. of an adduct of a glycerine initiated polyoxypropylene glycoi having an average molecular weight of about 2,700 with toluenediisocyanate, said adduct containing 9% by weight free NCO groups, said urethane modified DGEBA being prepared by the procedure described in U.S. 3,525,779 and wherein said urethane modified DGEBA had an average EEW of about 236.

I CTBN modified D GEBA is a diglycidyl ether of bisphenol A having an average EEW of about 189 modified by reaction with 10 phr. of a carboxyl containing butadieneacrylonitrile copolymer containing about 18.6% acrylonitrile and having an average molecular weight of about 2 000 said reaction conducted in the presence of about 1% by weight 0! D P 30 as a catalyst and said CTBN modified DGEBA had an average EEW of about 232.

8 The accelerator was a composition containing 82% dieyandiamide, 17% 2-methyl imidazole and 1% by weight of colloidal silica.

4 S=Stoichiometrlc quantity.

5 I=Example of the present invention.

C Comparative experiment.

TABLE II Sample No.

Cure schedule and property A-I A-C B-I B-C Lap shear strength, psi

C./60 min 5, 200 1 0 3, 950 l 0 120 G./18 hours I N .D. 4, 620 ND. 5, 200 Peel strength, p.s.i.:

120 C./45 min 1 98 l 0 I 52 l 0 120 C./18 hours N .D. 45 ND. 90

1 Sample did not cure. 1 N .D.=not determined. 1 Average of 3 test specimens.

5 6 for the formulation of the invention (A-I). When the M d comparative CTBN modified formulation (B-C) was heated sufliciently long, both shear and peel strengths are Measured M d pafter better than the same properties of the formulation of the $3 gig invention (B-I). Nevertheless, these properties for the 5 9 formulation of the invention are acceptable and they were g fg if j 558 "935 56 obtained in a much shorter cure time.

EXAMPLE 5 Epoxy resin powder coatings were prepared employing the curing agent of the present invention and also from other curing agents for comparative purposes. The powder oa ing formulation was as follows: 661 (A80) 125 300 grams of an epoxy resin of the diglycidyl ether of Dicy ndi mide t phr.) 4 bisphenol A having an average EEW of about 00, Dlmethylformamrde 15 90 grams of a mixture containing 90% of the above EM 15 800 EEW epoxy resin and 10% of Modafiow (a co- Action? 55 polymer of ethylhexyl acrylate and ethyl acrylate) a a Benzyldimethyl amine (0.3 phr.) 0.3

flow control agent. 192 grams of TiO pigment.

Formulation D includes strong solvents such as dimethylformamide (DMF) to prevent precipitation of the DICY. The biguanide formulation contains no DMF and yet showed no evidence of precipitation over a one-month storage period at room temperature.

To show latency or drift in cure rate with age stroke TABLE III Equivalents of curing MDA/DICY 1 MO (EA/DICY DMPB G DICY" 4 agent per equivalent of epoxy resin A 5 (percent) B a (see) A (percent) B (sec.) A (percent) B (sec.) A (percent) B (sec.) 22 240 20 230 75 80 70 75 22 810 21 420 78 315 96 810 38 810 27 900 85 330 81 705 50 960 7 N.D. N.D. N.D. N.D. 57 900 58 600 N.D. N.D. N.D. N.D N.D. N.D.

l MDA/DICY is bis-(4-big'uanidyl phenyl) methane as prepared by a procedure similar to Example 1. 2 MOOA/DICY is bis(4-biguanidyl-Q-chlorophenyl) methane as prepared by a procedure similar to Example 2. a DMPB G is 2,6 dimethylpheny1 biguanide employed for comparative purposes (as described in U.S. 3, 1

4 DICY is an accelerated composition containing 82% 63 ,149). t dicyandiamide, 17% 2-methylimidazo1e and 1% colloidal silica (all y w 6 A is the 20 gloss properties as determined by the procedures of AS'IM D-523. The values are reported as percent gloss retention.

B is the resistance to methylethyl ketone as determined by submerging the cured coating under a small watch glass containing a wooden spatula to test for softening and breakage of the film. The values given are times (in seconds) submerged required for softening and breaking.

MEK for a given number of seconds and then rubbing the coating with 7 N.D. indicates the value was not determined. Norm-Cure schedule=175 for minutes.

The results in Table HI show that the curing agents of the invention can be used to control the gloss by varying the amount of curing agent (biguanide), whereas with the DICY and DMPBG the effect is fairly constant regardless of their amount. The formulations containing MDA/DICY and MOCA/DICY as curing agents are generally more solvent resistant than the comparative formulations using the curing agents of the art.

The curing agents (biguanides) of the present invention when used in varnish formulations have one or more of the following advantages over the use of dicyandiamide:

(1) better solubility in less polar solvents,

(2) can be used without accelerator,

(3) no crystallization in B-stage (or prepreg) resins,

(4) no aging needed as do accelerated dicyandiamide formulations,

(5) no precipitation from epoxy mixtures as do dicyandiamides, and

(6) improved latency.

EXAMPLE 6 A varnish solution using MDA/DICY was made up as follows: (Formulation C) cure gel time tests were performed on the two varnishes above. The results are shown in the following graphic representation:

-Formulation 0 stroke cure, seconds D.E.R. 661 (A80) 125 Dowanol EM 25 H C Acetone 60 O 0 i ani e (5.4 phr 5.4 oHcHi -0--+- o-crn-ofi cm This varnish was used to prepare multi-ply glass rein- CH:

forced electrical laminate board having the following properties: (Cure 1 hr. at 350 F.)

7 Dowanol EM is the registered trademark of The Dow Chemical Company for its brand of the monomethyl ether of ethylene glycol.

The preceding examples clearly demonstrate that the curing agents of the present invention (1) cure at lower temperatures than dicyandiamide; or

more quickly at same temperature;

(2) confer better solvent resistance to ordinary solvents than those compounds having only one biguanide group per molecule;

(3) make it possible to prepare semi-gloss epoxy resin coatings which are not possible from compounds having only one biguanide group per molecule and which are not obtainable with dicyandiamide at low equivalents of dicyandiamide, i.e. below about 1.6 eq. of dicyandiamide/eq. of epoxy resin;

(4) possess better solubility in simple mixtures of less polar solvents;

(5) while accelerators may be used they are not required;

and

(6) have better shelf life than formulations known to the art.

We claim:

1. An epoxy resin composition curable upon heating to an insoluble infusable product which comprises in admixture (A) an epoxy resin having more than one glycidyl ether group and (B) a compound of the formula selected from the group consisting of (I) X x H1NC-NHCNH (A); NH-O-NH-G-NH:

NE E I IH I and wherein A is a divalent hydrocarbon radical having from 1 to about 6 carbon atoms,

each X is independently hydrogen, chlorine or bromine, and x has a value of 0 or 1, wherein the ratio of B:A is such as to provide an amine hydrogenzepoxide equivalent ratio of from about 0.8:1 to about 2:1.

2. The epoxy resin composition of claim 1 wherein Component B is a compound represented by Formula I.

3. An epoxy resin composition of claim 1 wherein in Component B, A is CH X is hydrogen and x is l.

4. An epoxy resin composition of claim 1 wherein A is --CH at least one X on each aromatic ring is chlorine and x is 1.

5. The thermoset product resulting from heating the composition of claim 1.

6. The thermoset product resulting from heatin the composition of claim 3.

7. The thermoset product resulting from heating the composition of claim 4.

8. The composition of claim 1 wherein an amine curing accelerator is employed.

9. The composition of claim 8 wherein the accelerator is an imidazole compound.

10. The composition of claim 9 wherein the imidazole compound is Z-methyl imidazole.

References Cited UNITED STATES PATENTS 3,631,149 12/1971 Gempeler 260-47 WILLIAM SHORT, Primary Examiner US. Cl. X.R.

117- 124 E; 161-185; 260-2 N, 32.6 R, 32.8 Ep, 33.2 Ep, 37 Ep, 47 EC, 49, 79, 565, 830 TW, 836 

